With an increasing number of U.S. states implementing multi-tiered systems of reading support in schools, educators require validated screening measures to identify students at risk for reading difficulties and inform reading instructional practices. This study evaluates the utility and validity of a new measure developed as part of the Dynamic Indicators of Basic Early Literacy Skills (DIBELS) literacy suite, First Sound Isolation (FSI). FSI measures students' ability to identify and pronounce the first sound they hear in words spoken aloud. A sample of 129 kindergarten students from four schools in the Pacific Northwest were administered FSI, DIBELS measures, and the Comprehensive Test of Phonological Processing (CTOPP-2) at the beginning of the 2021-2022 school year, and the CTOPP-2 and DIBELS measures at the end of the year. Descriptive statistics, concurrent and predictive correlations, Receiver Operating Characteristic curve analyses, and regression results indicate that FSI demonstrates smaller floor effects than do previous DIBELS measures of first sound and phonemic segmentation awareness, as well as strong internal consistency, concurrent validity, and predictive validity. FSI thus shows promise as an early measure of phonological awareness that, based on the current analyses, provides relevant and unique instructional information for educators.